1. Field of the Invention
The present invention relates to a press machine and a method of manufacturing pressed products, and particularly to an improvement for reducing mutual interference between a plurality of sets of molds to enhance the processing accuracy.
2. Description of the Background Art
FIG. 6 is an explanation diagram showing the structure of a conventional press machine as a background of the invention. This machine 151 has a bottom base 71 installed on the floor, a pair of supports 75a and 75b uprightly provided on the bottom base 71, and a top base 72 supported on the supports 75a and 75b. The bottom base 71, supports 75a and 75b, and top base 72 fixedly coupled to each other form a frame stand 86. A pair of fixed molds 73a and 73b are fixed on the bottom base 71. Fixed on the top base 72 are a pair of a (first) servo motor 76a and a (second) servo motor 76b. 
The servo motors 76a and 76b are respectively in mesh with ball threads 77a and 77b, which rotate to individually drive the ball threads 77a and 77b in the vertical direction. Moving molds 74a and 74b are fixed at the lower ends of the ball threads 77a and 77b, respectively.
The moving molds 74a and 74b are located right above the fixed molds 73a and 73b to face the fixed molds 73a and 73b, respectively. The servo motors 76a and 76b rotate in the normal rotation and reverse rotation directions to move the moving molds 74a and 74b in the mold-closing direction (i.e. downward) and in the mold-opening direction (i.e. upward).
The servo motors 76a and 76b are supplied with current (i.e., electric current) from a (first) servo amplifier 78a and a (second) servo amplifier 78b, respectively. The servo amplifiers 78a and 78b are individually controlled by an amplifier controlling unit 85, so that the magnitudes of currents supplied to the servo motors 76a and 76b are controlled individually. The amplifier controlling unit 85 includes a CPU 80 and a pulse generator 79.
FIG. 7 is a block diagram showing the inside structure of the servo amplifier 78a, which is representative of the servo amplifiers 78a and 78b. The servo amplifier 78a is supplied with a directing value X0 related to the operating position of the servo motor 76a (i.e. the rotating position of the rotor) from the pulse generator 79 and a measured value X related to the operating position of the servo motor 76a from an encoder 90.
As shown in the timing chart of FIG. 8, the directing value X0 is represented by the number of pulses along the time series. A normal rotation directing signal CW is outputted in pulse form when directing that the servo motor 76a should operate in the normal rotation direction, and a reverse rotation directing signal CCW is outputted in pulse form when directing that it should operate in the reverse rotation direction. The cumulative value of the difference between the number of pulses of the normal rotation directing signal CW and the number of pulses of the reverse rotation directing signal CCW corresponds to the directing value X0 related to the operating position of the servo motor 76a. 
The rate of change of the directing value X0 corresponds to the target value of the operating speed of the servo motor 76a (i.e. its rotating speed), which is proportional to the pulse frequency as shown in FIG. 8. The encoder 90 outputs pulses of the same form in correspondence with the amount of operation of the servo motor 76a (i.e. the amount of rotation of the rotor).
Referring to FIG. 7 again, the subtracter 91 calculates the difference between the directing value X0 and the measured value X and outputs the calculated value as a positional deviation xcex94X. The amplifier 92 amplifiers the positional deviation xcex94X. The subtracter 91 and the amplifier 92 form a position controlling unit. The F/V converter 97 converts the rate of time change in the measured value X, i.e., the frequency of the pulses representing the measured value X to a voltage signal. The subtracter 93 calculates the difference between the output signal from the amplifier 92 and the output signal from the F/V converter 97 and outputs the calculated value as a speed deviation xcex94S. The amplifier 94 amplifies the speed deviation xcex94S. The subtracter 93, amplifier 94 and F/V converter 97 form a speed controlling unit.
The output signal from the amplifier 94 is inputted to a current amplifier 96. The current amplifier 96 amplifies the input signal and supplies a current I proportional in magnitude to the input signal to the servo motor 76a. Thus the current I is controlled so that the measured value X follows the directing value X0 at speed proportional to the difference between the measured value X and the directing value X0 . The CPU 80 shown in FIG. 6 executes arithmetic processing and the directing value X0 is outputted through the pulse generator 79 on the basis of the value calculated in the arithmetic processing. The operation of the servo motor 76a is thus controlled.
FIG. 9 is a flowchart showing the procedure of the arithmetic processing performed by the CPU 80. When the arithmetic processing is started, first, the processings in steps S51 and S52 are simultaneously executed. Specifically, the servo motors 76a and 76b are driven to return to the origin (the initial position). This processing is continued until they have returned to the origin (step S53), and the process moves to steps S54 and S55 after it is finished. When they have returned to the origin, the moving molds 74a and 74b are positioned at the standby position separated above the fixed molds 73a and 73b. 
In the following steps S54 and S55, the servo motors 76a and 76b are driven to perform weighting operation. Then the moving molds 74a and 74b move in the mold-closing direction to respectively hit on the fixed molds 73a and 73b, and they are further pressurized for the press work. Steps S54 and S55 are simultaneously executed. These processes are executed until the press work is completed (step S56). When the press work has been finished, the process moves to steps S57 and S58.
In steps S57 and S58, the servo motors 76a and 76b are driven to perform withdrawing operation. Then the moving molds 74a and 74b move in the mold-opening direction to return to the standby position. The steps S57 and S58 are carried out at the same time. These processes are continued until they return to the standby position (step S59). When they have returned, the process moves to steps S54 and S55 again. The above-described processes are repeated to repeatedly carry out the press work.
FIG. 10 is a flowchart showing the internal flow in step S54, which is representative of steps S54 and S55. Similarly, FIG. 11 shows a flowchart showing the internal flow in step S57, which is representative of steps S57 and S58. FIG. 12 is a timing chart showing variations in the target value of the operating speed (i.e. the changing rate of the directing value X0), the positional deviation xcex94X, and the torque of the servo motor 76a that are caused in the weighting operation of step S54 and the withdrawing operation of step S57. Now, referring to FIGS. 10 to 12, the weighting operation and withdrawing operation of the machine 151 will be described.
When the weighting operation based on the processing in step S54 is started, first, the moving mold 74a is driven to move in the mold-closing direction at high speed (step S61). At this time, the target value of the operating speed first increases from zero, stays at a high value when the directing value X0 reaches a given reference value, and then decreases when the directing value X0 reaches another reference value. Subsequently, the target value of the operating speed is maintained at a low value (step S62).
The reference values for defining the operating positions at which the target value of the operating speed is changed are previously set through teaching performed prior to the processing in FIG. 9. The reference value defining the timing for changing from the high-speed moving operation based on step S61 to the low-speed moving operation based on step S62 is set so that the moving mold 74a is located at such a position that it does not abut on the fixed mold 73a when the directing value X0 reaches that reference value. Hence the moving mold 74a moves at high speed from the standby position toward the fixed mold 73a, whose speed decreases before it hits the fixed mold 73a, and then the moving mold 74a moves at low speed toward the fixed mold 73a. This reduces the impact produced when the moving mold 74a and the fixed mold 73a hits on each other.
The moving mold 74a hits on the fixed mold 73a at a certain point of time in the low-speed moving operation. While the moving mold 74a moves at speed approximately equal to the target value until it hits on the fixed mold 73a, it cannot maintain the speed corresponding to the target value after hitting. Accordingly, after hitting, the positional deviation xcex94X increases. Then the speed deviation xcex94S increases accordingly and the current I increases. As a result, the torque of the servo motor 76a increases. That is to say, the moving mold 74a is pressurized against the fixed mold 73a with an increasing pressing force.
After that, when the directing value X0 reaches another reference value, the operating-speed target value decreases toward zero. Then the process moves to step S63 and the operating-speed target value is maintained at zero. That is to say, the directing value X0 is held at a constant value. At this time, the moving mold 74a is pressed against the fixed mold 73a by a constant pressing force. The press work is carried out throughout from the beginning of pressing to the standing-still operation. The standing-still operation is ended when a previously set certain time has elapsed and the process moves to step S57.
In step S57, the moving mold 74a is driven to move at high speed in the mold-opening direction (step S71). During this operation, the operating-speed target value first increases from zero, stays at high value when the directing value X0 reaches a given reference value, and then decreases to zero when the directing value X0 reaches another reference value. The number of pulses of the reverse rotation directing signal CCW outputted as the directing value X0 in the high-speed withdrawing operation based on step S57 is equal to the number of pulses of the normal rotation directing signal CW outputted in step S61 (high-speed moving operation) and step S62 (low-speed moving operation). Then the pressing force applied to the moving mold 74a is quickly released and thereafter the moving mold 74a returns to the standby position at high speed.
The conventional machine 151 operates as described above to realize efficient press work while reducing impact between the moving molds 74a and 74b and the fixed molds 73a and 73b. 
However, since the two fixed molds 73a and 73b and the two servo motors 76a and 76b are provided on the single frame stand 86, the conventional machine 151 has the following problems. FIGS. 13 to 16 are timing charts used to explain the problems. In FIGS. 13 to 16, the speeds (a) and (b) represent the moving speeds of the moving molds 74a and 74b and the loads (a) and (b) represent the pressing forces applied to the moving molds 74a and 74b, respectively.
As stated above, the CPU 80 sends the directing value X0 to the servo amplifiers 78a and 78b so that the moving molds 74a and 74b arrive at the fixed molds 73a and 73b at the same time in the weighting operation. However, because of deflections of the bases 71 and 72, difference in capability between the servo motors 76a and 76b, slight errors in the transmission mechanism from the servo motors 76a and 76b to the moving molds 74a and 74b, and some other reasons, the moving molds 74a and 74b do not always arrive at the fixed molds 73a and 73b at the same time.
For example, as shown in FIG. 13, when the moving mold 74a arrives at the fixed mold 73a earlier than the moving mold 74b arrives at the fixed mold 73b, the moving mold 74b arrives at the fixed mold 73b after the moving mold 74a has arrived at the fixed mold 73a, in which case an excessive pressing force is applied to the moving mold 74a in the period before the pressing force to the moving mold 74b increases to a certain extent. This excessive load serves as a factor that reduces the processing accuracy in the pressing work.
Furthermore, using the machine 151 in a long time will cause deformation of the bases 71 and 72, variations in the characteristics of the servo motors 76a and 76b, wear of the transmission mechanism, and the like. Even if the simultaneous arrival is maintained, the deformation, variations, wear, etc. of the parts of the machine produced in long time use may cause inequality in pressing force between the moving molds 74a and 74b, as shown in FIG. 14. This inequality serves to reduce the accuracy of the press work, too.
Moreover, in the withdrawing operation, the moving molds 74a and 74b may separate from the fixed molds 73a and 73b at different points of time because of deflections of the bases 71 and 72, difference in capability between the servo motors 76a and 76b, slight errors in the transmission mechanism from the servo motors 76a and 76b to the moving molds 74a and 74b, and other reasons. For example, when the moving mold 74a separates from the fixed mold 73a earlier than the moving mold 74b separates from the fixed mold 73b as shown in FIG. 15, an excessive pressing force is applied to the moving mold 74b in the period from when the moving mold 74a starts withdrawing to when the moving mold 74b withdraws to some extent. This excessive load serves as a factor that reduces the accuracy of the press work, too.
Further, in the machine 151, the above-mentioned teaching is carried out individually to the two servo motors 76a and 76b. Specifically, the reference values for the directing value X0 directing the servo amplifier 78a and the reference values for the directing value X0 directing the servo amplifier 78b are separately set. The CPU 80 sends the directing value X0 individually to the servo amplifiers 78a and 78b while referring to the reference values set in this way. It is thereby attempted to improve the processing accuracy.
However, as shown in FIG. 16, when the reference values are set so that predetermined target load (pressing force) can be obtained through teaching (a) to the servo motor 76a and teaching (b) to the servo motor 76b that are separately performed, the pressing forces applied to the moving molds 74a and 74b may become lower than the target value in the following processing shown in FIG. 9. This is caused because the magnitude of deflection (the amount of deflection) occurring in the bases 71 and 72 differs between when the pressing force is applied to one of the moving molds 74a and 74b and when it is simultaneously applied to both.
As described above, the conventional press machine in which a plurality of sets of molds are coupled to a common frame stand has the problem that improvement of pressing accuracy is hindered because of mutual interference between the plurality of sets of molds.
A first aspect of the present invention is directed to a press machine having a plurality of fixed molds and a plurality of motors provided on a common stand, wherein the plurality of motors individually drive moving molds respectively in pairs with the plurality of fixed molds to perform press work. According to the present invention, the press machine comprises: a plurality of amplifiers for passing current individually through the plurality of motors; and an amplifier controlling portion for individually controlling the plurality of amplifiers to realize weighting operation of moving the plurality of moving molds in mold-closing direction and pressing the plurality of moving molds respectively against the plurality of fixed molds and withdrawing operation of moving the plurality of moving molds in mold-opening direction. Each of the plurality of amplifiers comprises a control portion for calculating an amount of current to be passed through a corresponding one of the plurality of motors so that a measured value of operating position of the corresponding motor follows a directing value, and a torque control portion for sending the amount of the current calculated by the control portion to the corresponding motor while limiting the same so that torque of the corresponding motor does not exceed a limit value, wherein in the weighting operation, the amplifier controlling portion further advances the directing value for each of the plurality of amplifiers in the mold-closing direction after the torque reaches the limit value.
Preferably, according to a second aspect of the present invention, in the press machine, in the weighting operation, the amplifier controlling portion advances the directing value for each of the plurality of amplifiers in the mold-closing direction before the torque reaches the limit value and lowers rate of change in the directing value before corresponding pair of the moving and fixed molds come in contact.
Preferably, according to a third aspect of the present invention, in the press machine, in the weighting operation, the amplifier controlling portion raises up the rate of change in the directing value for each of the plurality of amplifiers after the torque reaches the limit value.
Preferably, according to a fourth aspect of the present invention, in the press machine, in the weighting operation, the amplifier controlling portion lowers the limit value for each of the plurality of amplifiers at the same time as lowering the rate of change in the directing value before the corresponding pair of the moving and fixed molds come in contact.
Preferably, according to a fifth aspect of the present invention, in the press machine, in the withdrawing operation, the amplifier controlling portion advances the directing value for each of the plurality of amplifiers in the mold-opening direction while maintaining the limit value until corresponding pair of the moving and fixed molds open by a given amount or more, and then raises the limit value.
A sixth aspect of the present invention is directed to a method of manufacturing pressed products, and the method manufactures the pressed products by performing press work by using the press machine.
According to the machine of the first aspect, the directing value is further advanced in the mold-closing direction after the torque reaches the limit value, so that the effect of mutual interference between the plurality of sets of molds can be absorbed to perform the press work with stable load. This enhances the accuracy of the press work.
According to the machine of the second aspect, while the directing value is advanced in the mold-closing direction in the weighting operation, the rate of change in the directing value is lowered before the molds come in contact, i.e., mold contact occurs, which improves the efficiency of the work while avoiding impact caused as the mold.
According to the machine of the third aspect, the rate of change in the directing value is raised up after the torque reaches the limit value, so that a state with highly stable load can be realized quickly. Accordingly, even if the plurality of sets of molds come in contact at different points of time, it is possible to more effectively avoid intensive application of excessive load to a part of the sets.
According to the machine of the fourth aspect, in the weighting operation, the limit value of the torque is lowered at the same time as the speed of movement of the moving molds is lowered before the mold contact, so that the load can be stabilized in the press work and the travel of the moving molds can be finished in shorter time, thus further improving the efficiency of the work.
According to the machine of the fifth aspect, in the withdrawing operation, the limit value of the torque is maintained until the molds open by a given amount or more, and then the limit value of the torque is raised. Accordingly, even if the plurality of sets of molds separate at different points of time, it is possible to more effectively avoid intensive application of excessive load to a part of the sets.
According to the manufacturing method of the sixth aspect, it is possible to obtain pressed products with excellent processing accuracy.
The present invention has been made to solve the above-described problems in the background art, and an object of the present invention is to reduce mutual interference between a plurality of sets of molds to provide a press machine and a pressed product manufacturing method with improved processing accuracy.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.